Polytetrafluoroethylene coating of anodized copper wire



March 15, 1960 H. F. PUPPOLO 2,928,776

POLYTETRAFLUOROETHYLENE COATING OF ANODIZED COPPER WIRE Filed June 25, 1956 2 Sheets-Sheet l RY F. PUPPOLO HEN INVENTOR.

-HIS ATT RNEYS Ma rch 15, 1960 H. F. PUPPOLO 2,928,776

POLYTETRAFLUOROETHYLENE COATING OF ANODIZED COPPER WIRE Filed June 25, 1956 2 Sheets-Sheet .2

72 Ra 7 u r HENRY 'F. PUPPOLO INVENTOR.

um; HIS ATTO NEYS POLYTE'I'RAFLUORQETHYLENE COATING OF AN ODIZED COPPER WIRE Henry F. Puppolo, North Adams, Mass, assignor to Sprague Electric Company, North Adams, Mass, a corporation of Massachusetts Application June 25, 1956, Serial No. 593,389

2 Claims. (Cl. 204-28) The present invention relates to the coating of polytetrafiuoroethylene resin on copper surfaces. This resin is an exceedingly difficult one to uniformly adhere to metal surfaces, particularly in very .small thicknesses. Even though so-called polytetrafluoroethylene primers have been suggested to improve the adhesion, their use does not produce coatings of sufficiently high quality.

Among the objects of the present invention is the provision of novel methods for improving the quality of the above coatings, as well as the resulting products.

The above as well as additional objects of the present invention will be more clearly understood from the following description of several of its exemplifications, reference being made to the accompanying drawings, wherein:

Fig. 1 is a more or less diagrammatic illustration of a wire-coating technique in accordance with the present invention; and

Fig. 2 and Fig. 3 are front and side views respectively of an arrangement for heat treating the coated wire to further improve its characteristics.

It has been discovered that the adhesion of polytetrafluoroethylene resin to copper surfaces is greatly improved if, before the coating is applied, these surfaces are subjected to an anodic electrolytic treatment for from /2 to 5 seconds in an aqueous solution of chromic acid having at least 1% chromic acid by weight, ata current density of from about 50 to about 500 milliamperes per square inch of surface.

It is believed that this treatment forms an exceedingly thin oxide film on the copper surface, and that this film effects the improvements. above treatment polytetrafiuoroethylene coatings do not adhere as well and furthermore show considerable nonuniformity.

In addition to the above, the polytetrafiuoroethylene coatings produced by the present invention are still further improved by a heat treatment of the resin-coated In any event, without the tes atent 0 F copper at 190 to 210 C. for from 20 to hours, and

such heat treatment which usually changes the appearance of the coating, gives less variation in appearance as compared with the coatings that are applied without the above anodic treatment. For this heat treatment it is particularly desirable to make sure that all parts of the coated surface are fairly uniformly treated. Thus, if the copper surface is that of a wire, and the wire is wound on a spool, somewhat better results are obtained if the spool is perforated so as to fully expose the mass of wound wire at a great many places.

Referring now to Fig. 1, there is here shown in diagrammatic form a complete set of coating steps for adherently applying a polytetrafluoroethylene resin layer to copper wire. A feed spool 10 supplies the wire 11 which is fed under idler roll 12 and through a succession of individual treating bath containers 14, 15, 16, and 17. All these containers may be identical, and are equipped with devices that permit the wire to pass through the container wall without causing any of the baths in these containers to leak out to any appreciable degree. As illustrated the containers have floors 20, on the internal face of which there is anupstanding thimble 22 integrally united to the floor. The thimble 22 is hollow to provide a passageway for the wire, and at the top of the thimble there is frictionally mounted a rubber nipple 24 of the kind used in connection with babies nursing bottles. The base of the nipple is preferably arranged so that it has to be stretched over the top of the thimble and the elasticity of the rubber holds the nipple securely in place. At its top, the nipple has a perforation 26 which can be smaller in diameter than the wire being treated so that the lip of the perforation seals against it. The wire emerging from the perforation continues up through the balance of the individual containers and in this way passes through all of them in series.

After container 17 the wire passes over pulley 28 and then under pulley 30 for another upward traverse, this time through an oven 32. Another set of reversing pulleys 34, 36 brings the wire to a coating container 38 and another oven 40. Container 38 is shown as identical to containers 14 to 17 inclusive, and oven 40 may be identical to oven 32. A similar set of reversing pulleys 42, 44 provides an additional traverse through another coating container 46 and a third oven 48. From here an idler roll 50 permits the wire to be wound on takeup spool 52. The wire is preferably impelled by connecting pulleys 23 or 34 to a driving mechanism, and equipping feed spool 10 with a brake to supply a uniform feed tension. The take-up spool 52 can be filled with a take-up mechanism to keep up with the'supply drive.

Before passing through the coating containers, the wire can advantageously be dipped'in a wetting agent as by providing a dip through 54 in such a position that the wire passing under reversing rolls 36 and 44 becomes wet by a body of wetting agent solution in these troughs.

In the embodiment of Fig. l, containers 14 and 15 are fitted with electrodes 60,62 respectively supported by a container wall for example, so that the wires Will pass through them. The apparatus of Fig. 1 is used by placing a cleaning bath such as a 15% by weight solution of sodium hydroxide inwater in container 14, a 6 /2% by weight solution of chromic acid in water in container 15, and ordinary tap water in containers 16 and 17.

t the same time, containers 38 and 46 are filled with aqueous suspension of polytetrafluoroethylene resin. Suitable suspensions are described in US. Patents 2,534,058, 2,478,229 and 2,562,117. Wetting agents are preferably of the anionic type as sodium lauryl sulfate.

The electrolytic action of the present invention is conveniently supplied by connecting electrode 60 to the positive terminal and electrode 62 to the negative-terminal of a source of D.C. electric power at a potential of about 5 to 20 volts.

With the above arrangement oxygen-free high-conductivity copper wire of AWG26 moving at a rate of 40 feet per minute provides a very effective coated product with 10 inches of wire exposed to the bath in containers 14 to 17 inclusive, the-current adjusted to milliamperes, oven 32 arranged to have a temperature varying from 700 degrees F. at the top to 400 degrees F. at the bottom, the wetting agent baths are 10% solutions by weight of sodium lauryl sulfate in water, the first coating bath is a primer bath described in example of US. Patent No. 2,562,117, oven 40 varies from 1400 degrees F. at the top to 700 degrees F. at the bottom, the coating bath in 46 is a polytetrafluoroethylene suspension having 36% resin solids by weight and /z% sodium lauryl sulfate by weight, and the oven 48 varying from 1300 degrees F. at the top to 800 degrees F. at the bottom.

The wire produced this way has a very uniform ap-. pearance and also shows a uniform adhesion of the resin coating, at least higher than a wire that passed through the same treatment with the electrode 62 short circuited to the wire.

It is generally desirable to acidify the polytetrafluoro ethylene coating suspensions as by adding concentrated hydrochloric acid to bring the pH to about 1.6.' For maximum adhesion, the bath in container 46 can be made of a mixture of equal parts of the primer suspension in container 38 and pure or primer-free polytetrafluoroethylene suspension. On the other hand, the Improveinent of the present invention are obtainable even when the pure or primer-free polytetrafluoroethylene T6813 is used in both containers 38, 46, andrwhere only one coating bath is used wtih either type of suspension.

The cleaning bath in container 14 can be of many different kinds including solutions of sodium orthosilicate, sodium metasilicate, trisodium phosphate, sodium carbonate, and mixtures of the above. The cleaning treatment in this container can be electrolytic or non-electrolytic. Shot-ting electrode 60 to the wire keeps electrolytic current from passing through the cleaning bath and thereby enables non-electrolytic cleaning. If the wire is cleaned when it is originally supplied to spool 10 for example, the cleaning container 14 can even be entirely omltted.

As indicated above, the chromic acid bath in container 15 should have at least 1% chromic acid. The presence of alkali metal chromate in the chromic acid bath, as by introduction of alkali, for example, by dragout from the cleaning bath, does no harm. In fact, a solution of sodium chromate or potassium dichromate can be used in place of the chromic acid inasmuch as the electrolytic action generates free chromic acid immediately on the wire. The electrode 62 should be relatively inert material such as stainless steel. Washing containers 16 and 17 can, if desired, be replaced by any other form of washing treatments such as merely causing a stream of tap water to wash down on the upwardly moving wire. The washing containers, if used, can have their water continuously replaced as by continually introducing fresh water and causing the contents to overflow into a waste line.

The oven 32 can be dispensed with, but it is preferred to have an oven treatment after each passage through a resin suspension.

With the smaller wire sizes, that is from No. 28 to 38 AWG for example, better results seem to be obtained with the anodic treatment somewhat lower than in the above example. An anodic current density of about 300 milliamperes per square inch, down to as low as 120 milliamperes per square inch, with the wire speed increased to the point where the anodic treatment only lasts for one second or even slightly less seems to be desirable in these cases.

It is preferred, especially with the larger wire sizes, to have the wires treated with a minimum of travel over reversing rollers or pulleys. As an extreme measure, all the containers and ovens can be arranged in a single line. Alternatively, and particularly where additional reversals are of no consequence, the wires can merely be passed in and out of the various baths in the manner shown at 54 for the wetting agent treatment. Where the nippled containers are used, the nipples are highly effective in reducing leakage at the site where the wire penetrates into the container. If desired, however, mercury seals as described in copending application Serial No. 371,439, filed July 31, 1953, can be used in place of the nipples, or mercury can be added to the nippled containers so that the mercury level is above the top of the nipples, thereby effecting even better sealing. Mercury should not be used to seal containers 14 to 17 inclusive, since it will amalgamate with the, copper and ruin the product.

'- I A feature of the present invention is that wires coated in the above manner are uniformly improved by a prolonged heat treatment such as that described earlier. To this and, best results are obtained by using a perforated type of spool for holding the wire during heat treatment.

' Figs. 2 and 3 are views of a suitable spool for this purpose. The spool is made ofnickel plated steel, and has side walls 71, 72 connected together by an internal tube 74 and the external cylindrical spool floor 76. These elements can be held together in any convenient manner as by crimping the ends of tube 74 against the outer face of wall disc 78, and crimping an outer portion of these discs to the turned up margins of the spool floor 76. As shown in floor 76, the tube 74 and end disc 78 are all perforated with holes. The convenient sizes of these holes is about of an inch in diameter, with holes'spaced from each other about A; of an inch apart. However, the advantages of the invention are obtained with holes that are only about ,5 of an inch in diameter. With larger sizes of wire, it would be simpler to use holes about A of an inch or more in diameter.

In place of nickel plated steel ordinary steel can be used, in which event, the steel is preferably cleaned, lacquered, as with a standard cellulose acetate lacquer, and then heated for two minutes at 650 degrees C. This gives the spool a carbonized finish which is satisfactory.

No. 28 gauge wire coated as in the above example, is very effectively improved by a 29 hour heat treatment in an air oven held at 205 degrees C. For larger sizes of wires, the time of treatment can be reduced to about 20 hours. If desired, the air of the oven can be circulated as by a blower with the air stream directed at the spool perforations. An inert gas such as nitrogen can be used as the oven atmosphere in place of air.

The process of the present invention is applicable for use with copper wire that is either round, square or polygonal in cross-section. It can also be used with copper in forms other than wire. Copper foil, copper sheet and even copper rods, either round or square in cross-section, can be so treated and will show the improved results. A minor amount of alloying ingredients, such as zinc, nickel, or the like, can also be present in the copper without detracting from the benefits obtained.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method of adhering a polytetrafiuoroethylene resin coating of uniform appearance on a copper wire which comprises the steps of cleaning the surface of a copper wire by passing through an aqueous bath, anodically treating the surface of the copper wire for a period from about /2 to 5 seconds by passing through an aqueous solution of chromic acid having at least 1% chromic acid by weight, at a current density of from about 50 to 500 milliamperes per square foot of wire surface to generate free chromic acid immediately on the wire surface and coat the surface with a thin oxide, preheating the anodically oxidized surface to a temperature of over 400 F. up to 700 F. to produce a uniformthin oxide, first coating the preheated and oxidized base surface with polytetrafluoroethylene suspensoid composition having an acid pH, second coating the first coated surface with a dispersion of finely divided particles of polytetrafluoroethylene resin, sintering the resin particles on the coated surface and uniformly improving the coated wire by subjecting the coated wire to a heat treatment of to 210 C. for from 20 to 50 hours to produce a coat of uniform adherence and uniform appearance on said wire surface.

2. A method of adhering a polytetrafluoroethylene resin coating of uniform appearance on a copper wire which comprises the steps of cleaning the surface of a copper wire by passing through an aqueous bath, anodically treating the surface of the copper wire for a period from about /2 to 5 seconds by passing through an aqueous solution of chromic acid having at least 1% chromic acid by weight, at a current density of from about 50 to 500 milliamperes per square foot of wire surface to generate free chromic acid immediately on the wire surface and coat the surface with a thin oxide, preheating the anodically oxidized surface to a temperature of over 400 F. up to 700 F. to produce a uniform thin oxide, first coating the preheated and oxidized base surface with polytetrafluoroethylene suspensoid composition having an acid pH, second coating the first coated surface with a dispersion of finely divided particles of polytetrafluoroethylene resin, sintering the resin particles on the coated surface and uniformly improving the coated wire by coiling the sintered coated wire on a perforated spool and subjecting the coated wire to a heat treatment at 190 to 210 C. for from 20 to 50 hours to produce a coat of uniform adherence and uniform appearance on said wire surface.

References Cited in the file of this patent UNITED STATES PATENTS Mason Oct. 13, Mason Oct. 13, Berry Aug. 9, MacKinzie Dec. 12, Osdal July 24, Robinson et al Apr. 22, Burnham July 28, Flynn et al Ian. 25, Hurd May 15, Hurd et a1. Aug. 13,

FOREIGN PATENTS Great Britain May 4, 

1. A METHOD OF ADHERING A POLYTETRAFLUOROETHYLENE RESIN COATING OF UNIFORM APPERANCE ON A COPPER WIRE WHICH COMPRISES THE STEPS OF CLEANING THE SURFACE OF A COPPER WIRE BY PASSING THROUGH AN AQUEOUS BATH, ANODICALLY TREATING THE SURFACE OF THE COPPER WIRE FOR A PERIOD FROM ABOUT 1/2 TO 5 SECONDS BY PASSING THROUGH AN AQUEOUS SOLUTION OF CHROMIC ACID HAVING AT LEAST 1% CHROMIC ACID BY WEIGHT, AT A CURRENT DENISTY OF FROM ABOUT 50 TO 500 MILLIAMPERES PER SQUARE FOOT OF WIRE SURFACE TO GENRATE FREE CHROMIC ACID IMMEDIATELY ON THE WIRE SURFACE AND COAT THE SURFACE WITH A THIN OXIDE, PREHEATING THE ANODICALLY OXIDIZED SURFACE TO A TEMPERATURE OF OVER 400* F. UP TO 700* F. TO PRODUCE A UNIFORM THIN OXIDE, FIRST COATING THE PREHEATED AND OXIDIZED BASE SURFACE WITH POLYTETRAFLOURETHYLENE SUSPENSOID COMPOSITION HAVING AN ACID PH, SECOND COATING THE FIRST COATED SURFACE WITH A DISPERSION OF FINELY DIVIDED PARTICLES OF POLYTETRAFLOUROETHYLENE RESIN, SINTERING THE RESIN PARTICLES ON THE COATED SURFACE AND UNIFORMLY INPROVING THE COATED WIRE BY SUBJECTING THE COATED WIRE TO A HEAT TREATMENT OF 190* TO 210* C. FROM 20 TO 50 HOURS TO PRODUCE OF A COAT OF UNIFORM ADHERENCE AND UNIFORM APPEARANCE ON SAID WIRE SURFACE. 